The positioning of components is normally performed with the support of a setting element which serves the purpose of establishing a movement of a displaceable component opposite another component which, on its part, remains in the rest position during the displacement. In an examining system, the non-moved component during the displacement of the displaceable component can also be regarded as being a type of frame. For this reason, this designation is also adopted in the following.
The examining system in the sense of this application is a plant, an apparatus or a device that is suitable for performing research or analysis on any examination object with regard to at least one characteristic of a physical, chemical or biological nature. Such systems are frequently allocated to the field of the so-called scientific equipment construction. They are at least partly characterised by high-precision mechanical components, particularly where the exact displacement of displacement-capable components is concerned.
For setting elements which are required to function with a high-sensitivity resolution, piezo-electrically operated setting elements for example, which are also designated as piezoelements, are used as actuators whose expansion depends on an applied electric voltage. However, other setting elements are also known as such in the state of the art, for example hydraulically or inductively operated setting elements. With the known piezoelements, an actuating element actually causing a displacement of a component is, for example, moved to a pre-specified location by the setting of a voltage and remains solidly at that location. As piezo-elements have a hysteresis, meaning, the expansion at a certain voltage depends on the home position of the displacement motion and, beyond this, move for some time after the voltage has been switched off, also designated as “creeping”, the actual deflection of the piezoelement or the displaceable component is to be measured during the operation of the piezoelement with a voltage activation and, with the support of a closed loop regulation, it must be ensured that the expansion of the piezoelement is effected linearly to the applied voltage. The quality of the linearity in this case naturally depends on the type of the selected spacing sensor which can be, for example, a capacitive sensor, an LVDT (linear variable differential transformer) or a strain gauge. However, other spacing sensors are useable such as for example optical sensors. Errors occur regularly with known types where corrections are performed with software controls.
The component to be displaced can either be solidly joined to the actuating element, by means of adhesive sticking for example, or a joint is formed with which the displaceable component is joined to the other component which remains locally fixed-positioned during the displacement action. A solid body joint is used, for example. In this case, piezoelements can be clamped in between the parts of the solid body joint or can be fastened to this in another way in order to initiate the displacement movement by means of a projection or a retraction of the joint.
In addition to this, a step-up mechanism is frequently used in order to convert the drive movement produced by a drive apparatus of the setting element for the displacement of the displaceable component. Thus, displacement routes are obtained which are larger than the extent or a stroke of the drive movement itself.
In the sense of the present application, a direct drive for the displacement movement is involved if the displacement of the displaceable component is achieved from a home position into an end position wherein an actuating element coupled to the displaceable component is moved between a retracted position in which the actuating element is at least partially retracted and a projected position in which the actuating element is at least partially projected. Such a direct drive can, for example, be provided for with the support of the piezoelement.
In contrast to this, indirect drives are also known. This includes, for example, the displacement of the displaceable component wherein a setting element provided with a thread is coupled to the displaceable component and is displaced by the turning of a screw in the thread. In this way, the setting element is moved longitudinally along the screw. However, such indirect connections normally lead to reduced dynamic effects and increased inaccuracy of the positioning.
The usage of a direct drive of the known type, however, also leads to disadvantageous effects. The process of the actuating element as required for the position finding must be upheld in the end position. For a piezoelement this means that, because of the coasting after the shutoff of the voltage (creeping) the applied voltage still has to be varied. The deflection of the actuating element additionally involves a noise. For example, a piezoelement requires a voltage source whose noise can be minimised but can never be fully suppressed. This noise causes a fluctuation of the position of the displaceable component in the end position by a required value (setpoint). If the end position is additionally monitored with the support of a sensor whose signals are used for a control, the resolution of the sensor can also represent a limitation of the positioning accuracy.
Moreover, and by way of a vibration-capable system to which the actuating element also belongs, the component to be displaced remains joined to the frame in the end position also. If the component to be displaced is, for example, adhesively joined to the actuating element or fastened to it directly in any form, the actuating element is itself a spring-mass-system. If disturbing movements are then imposed onto the overall system, this will then lead to problems if the spectrum of these disturbances lies in the vicinity of or above a resonance frequency of the overall system. In this situation, relative movements of the components to one another then occur.
The problems as described occur particularly with measurement and analytic equipment, such as with a scanning probe microscope, for example. In this case, both the resolution and the dynamics of the microscope are limited. Setting elements are normally used for all three spatial directions in such a microscope. The inaccuracy of the setting elements due to the noise sources restricts the measuring possibilities. With a lateral scanning range of 100 μm×100 μm, for example, an atomic resolution is often not possible because the voltage sources of the piezoelements generate noise. In the case of a control, the noise of the sensor adopted for the spacing measurement is added to the control signal. Accordingly, the setting element has a certain local blurring which amounts to approximately 0.3 nm for the previously mentioned example. The dynamics are determined by the size of required piezoelements or a correspondingly selected step-up, so that the speed cannot be randomly increased for any desirable scanning range. If a higher resolution or a higher bandwidth is desired, the scanning range must then be reduced.